The invention relates to copolyamides prepared from terephthalic acid (TA) and/or isophthalic acid (IPA) or their respective esters or ester forming derivatives, and mixtures of diamines containing 5-methylnonamethylenediamine-1,9 (5-MNDA), optionally of technical grade, containing isomeric diamines, and p,p'-diaminodicyclohexylmethane or its derivatives.
U.S. Pat. No. 2,752,328 discloses polyamides prepared from terephthalic acid and diamines having a methyl-substituted hydrocarbon chain. The diamines used have terminal amino groups and a methyl-substituted hydrocarbon chain of 6-8 atoms, and the total number of C-atoms should not exceed the value obtained by subtracting the number of methyl groups from 10. The polyamides so prepared exhibit good textile properties, especially with respect to a high zero strength temperature and slight shrinkage. However, only small deparatures from the specified amines (see column 6, lines 58-65) result in polyamides and fibers which display undesirable properties. Thus, when, in Example 11,5-MNDA is used as the diamine for comparison, the polyamide and fibers prepared exhibited a shrinkage in boiling water of 78%, a zero-strength temperature of 160.degree. C. and tenacity of 1.9 g/denier.
German Offenlegungsschrift No. 23 12 218 describes linear terephthalamide polymers which consist at least principally of repeating units of the formula ##STR2## wherein, in this formula, R=5-methylnonamethylene, 2,4-dimethyloctamethylene, and 2,4,6-trimethylheptamethylene, and at least 50%, preferably at least 70%, and especially preferably at least 80% of the R groups are 5-methylnonamethylene groups. Although according to the Offenlegungsschrift the polymers can contain up to 20% by weight of other repeating structural units, in general the polymers are comprised of at least 90% by weight, preferably at least 95% by weight, and most preferably at least 98% by weight of repeating structural units of the C.sub.10 -diamine terephthalamide defined in the formula above. The linear polymers which consist essentially of the C.sub.1-10 -diamine terephthalamide of this formula as the sole repeating structural unit are distinguished by especially good properties. Among these properties are improved values for the shrinkage in boiling water, the zero strength temperature, and the tenacity. Such polamides are here characterized as linear since they are fusible and soluble in the usual solvents for polyamides, such as m-cresol, formic acid, and the like. Polyamides of this structure, contrary to the assertion of U.S. Pat. No. 2,752,328, are also well suited to the preparation of fibers.
A disadvantage of the polyamides disclosed in German Offenlegungsschrift No. 23 12 218 prepared from 5-MNDA and TA is their relatively low glass transition temperature, which is at most 120.degree. C.